I'm sorry, I don't know any font with such a /O but I can't state it's custom made. May be it is as simple as "generic extended sans like Helvetica/Akzidenz Grotesk with a custom /O".
What I can propose though:
- the logo itself, freely available to download as vector: http://www.brandsoftheworld.com/logo/only-denim
- some similars: Foundry Monoline, NeoGram, Sui Generis (which comes very close, for the /NLY part at least), http://Forza
- a possible track to follow: have a closer look to the /® (register mark), the external circle and /O look like to share the same design. /R reminds me once again of Foundry Monoline. So my last call: custom job based on Monoline.